In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of an aqueous mixture, for example, comprising water and one or more organic materials such as a monohydric alcohol, a polyhydric alcohol, or the like.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. There are generally two types of ink-receiving layers (IRL's). The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling, which non-porous coating absorbs ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. Typically, this coating is optically transparent and very smooth, leading to a very high gloss “photo-grade” receiver. However, this type of IRL usually tends to absorb the ink slowly into the IRL and, consequently, the imaged receiver or print is not instantaneously dry to the touch.
The second type of ink-receiving layer or IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open interconnected pores of the IRL, substantially by capillary action, to obtain a print that is instantaneously dry to the touch. Typically the total interconnected inter-particle pore volume of porous media, which may include one or more layers, is more than sufficient to hold all the applied ink forming the image.
Basically, organic and/or inorganic particles in a porous layer form pores by the interstitial spacing between the particles. The binder is used to hold the particles together. However, to maintain a high pore volume, it is desirable that the amount of binder is as low as possible. Too much binder would start to fill the pores between the particles or beads, which will reduce ink absorption. Too little binder may reduce the integrity of the coating causing cracking. Once cracking starts in an inkjet coating, typically at the bottom of the layer, it tends to migrate throughout the layer.
A porous IRL that is glossy usually comprises at least two layers, a base layer and a glossy image-receiving layer. When coated on plain paper, the base layer is laid down underneath the glossy image-receiving layer, that is, the base layer is located between the image-receiving layer and the support. In order to provide a smooth, glossy surface on the image-receiving layer, special coating processes are often utilized, such as cast coating and film transfer coating. Calendaring, with heat and pressure, is also used in combination with conventional blade, rod, or air-knife coating on plain paper to produce a glossy image-receiving layer.
For porous coated papers, one of the main functions of the base layer is to provide a sump for the ink fluids. As the quality and density of inkjet images increases, so does the amount of ink applied to the inkjet receiver. For this reason it is important provide sufficient void capacity in the base layer. Although many types of inorganic or organic particles can be used in the base layer, calcium carbonate particles has been found useful to provide enough void capacity when coated on a substrate. Calcium carbonate can be natural (ground) or synthetically made (precipitated) and can come in a variety of sizes and shapes.
A requirement for porous inkjet paper manufacture is that the coating compositions used to make the porous layers are compatible with modern coating and drying equipment. Modem coating and drying methods in the paper coating industry have evolved to high levels of efficiency. These methods include blade and rod coating, mentioned above, that are capable of high coating speeds that contribute to manufacturing efficiency. Coating compositions for porous layers (sometimes referred to as “coating colors”) comprise particles such as, for example, pigment dispersions, wherein pigments refer to inorganic particles. The coating compositions comprise a high concentration of solids for drying efficiency and, in fact, the common coating methods will not work unless the particles are sufficiently concentrated. Dryers are typically gas fired and operate at temperatures in excess of 200° C. As a result of the high solids concentrations and hot dryers, energy usage and dryer lengths are minimized.
In general, inkjet base-layer coatings typically have high dry coverage compared to common coated papers. For example, typical dry coverage for inkjet base-layer coatings is 10 grams per square meter or more (≧10 g/m2). The base-layer coatings also have to be highly porous to absorb the aqueous ink carrier solvents deposited during inkjet printing.
For rod coating, dry coverage increases with coating solids concentration. Thicker layers using self-metered methods such as rod and blade require relatively higher solids concentrations, whereas thinner layers may use coatings with relatively lower solids concentrations. Typically the solids concentration of coating compositions used for the rod and blade coating of base layers in inkjet media is in the range of 50% to 70% by weight. The solids concentration must be high enough that the particle concentration approaches close packing where flow cannot occur. The viscosity of such coatings typically falls rapidly as shear rate is increased and plateaus at high shear rates to a viscosity value called the high shear viscosity. For rod coating, the high shear viscosity is typically in the range of 0.1 to 1 poise.
In the prior art, the necessary porosity for a porous layer has been achieved by using microporous pigments at least in part, that is, pigment particles that are themselves porous. Silica gels and fumed aluminas are examples of microporous pigments. However, these materials can be costly and difficult to disperse at high solids. In dispersion, microporous pigments absorb and immobilize part of the liquid phase with the result that the viscosity of the coating composition greatly increases. At particle concentrations approaching close packing, the liquid in the micropores represents a drying penalty. In addition, formulations too high in viscosity are impractical to handle. Pumping, deaerating, filtering and mixing are examples of standard operations that can be compromised by an overly high viscosity. Microporous pigments can also be difficult to handle in the dry state and to disperse. As a consequence, the quantity of microporous pigments that can be used in a coating composition can be limited. While these operating difficulties can be relieved by adding enough water, the coating composition may be made too dilute for coating and the desired drying efficiency, especially for a base layer.
An alternative way to produce highly porous coatings is to employ structured pigments in which the dispersed particles have low or no internal porosity. These may less expensive than microporous pigments. Structured pigments have a non-spherical morphology that does not allow dense packing in the dried coating. In the coating composition, structured pigments immobilize less water than microporous pigments and so do not have the inherent drying penalty. They may be able to be dispersed at the required concentrations without causing unmanageable viscosity.
Precipitated calcium carbonate (PCC) is an example of a structured pigment that can provide high porosity in inkjet coatings. For example, precipitated calcium carbonate having scalenohedral morphology has been used to provide absorption of inkjet-printing inks.
U.S. Pat. No. 6,150,289 to Chen discloses a coating composition intended for a matte-grade inkjet paper comprising engineered calcined clay dispersed with a cationic polymer and compares this with a composition comprising scalenohedral precipitated calcium carbonate (PCC) particles, binders and crosslinker. At relatively low solids (less than 35%), no rheology problems are mentioned and no suggestion of mixing different morphologies of precipitated calcium carbonate (PCC) particles is made.
U.S. Pat. No. 5,783,038 to Donigan discloses an inkjet recording element coated with precipitated calcium carbonate (PCC) particles milled and heat-aged in the presence of an organo-phosphonate compound. The precipitated calcium carbonate particles may be selected from scalenohedral, acicular, prismatic or rhombohedral morphology. No teaching is provided regarding preferred particle morphology, a mixture of particle morphologies, or coatability on a manufacturing scale at high solids concentration with rod or blade coating apparatus.
U.S. Pat. No. 6,379,780 to Laney discloses a two-layer film laminate comprising an impermeable base polyester layer and an absorbing top polyester layer comprising a filler of a scalenohedral form of precipitated calcium carbonate (PCC). In this example, the recording elements are produced by an entirely different process comprising extrusion, stretching, and tentering, to generate voids, rather than being coated from aqueous coating composition and dried.
U.S. Pat. No. 6,689,430 to Sadasivan discloses an inkjet recording element comprising a base layer comprising prismatic particles of precipitated calcium carbonate (PCC) and silica gel.
U.S. Pat. No. 5,879,442 to Nishiguchi et al. describes a method of preparing aqueous slurries of mixtures of precipitated calcium carbonate and ground calcium carbonate for coatings of papers. The relative weight proportions of precipitated and ground calcium carbonate particles are from 20:80 to 80:20.